This invention relates to a system for evaluating a slurry and particularly to such a device for measuring the concentration of ash producing constituents of a coal slurry.
In a typical coal preparation plant, coal is processed in an aqueous medium to permit the separation of carbonaceous material from ash forming minerals in unit operations that exploit differences in physical and/or chemical properties between coal (carbonaceous) particles and impurities. Processors of coal need to evaluate the slurry periodically to ascertain its composition. In particular, the concentration of ash forming minerals in the coal is important to the user and is monitored in order to control the key process variables so that the desired specifications can be met in the end product or "clean" coal.
The standard method of determining ash concentration is prescribed by the American Society for Testing and Materials (ASTM) standard number 3174, which requires a coal sample to be combusted, and an ash determination made to evaluate coal composition. This evaluation process requires a period of time on the order of five hours between initial sample taking and data reduction. Due to this lengthy time requirement, the measurements provided by this method cannot be used in a closed loop process control system as a means of controlling slurry composition or monitoring plant operation.
In order to permit control over plant operations in a more timely manner, various approaches have been considered to enable more rapid assessment of coal slurry composition. One concept involves using detectors which evaluate the slurry by its absorption of radiation in one or more energy bands which is related to its composition. For these systems, a sensor is placed either directly into a slurry sample or mounted on a slurry transport conduit. One particular problem with ash monitoring systems using radiation type detectors is the influence of entrained gas bubbles and dissolved gases which interfere with evaluation accuracy since they generate signal bias, particularly if radiation absorption measurements are taken. The presence of bubbles also adversely affects the measure of the density of the sample. Another shortcoming of such systems is attributable to the fact that they do not sense a homogeneous mixture of slurry or the entire cross section of the slurry, thereby giving rise to additional sources of error.
In one prior art system according to U.S. Pat. No. 4,282,434, issued to Lyman, radiation absorption measurements are taken as a means of determining ash concentration. Lyman also attempts to reduce the effects of entrained gas bubbles within the sample by collapsing them by applying a positive pressure to the sample causing the gas to dissolve into the aqueous medium of the sample. Although the system of Lyman would produce a more accurate density value for the sample by eliminating entrained gases, it does not eliminate the air molecules from the sample. Accordingly, dissolved air within the sample would influence the attenuation of radiation passing through the sample, thereby giving rise to a source of error in the ash determination.
In another prior art system described by U.S. Pat. No. 3,031,571, issued to Fearon, a dry sample is mixed with a stream of liquid mercury. A vacuum is applied to a sample to cause the sample and mercury to be pumped and passed across a sensor. The sensor measures excited X-rays caused by electron bombardment. The Fearon system would not provide an accurate means of evaluating ash concentration and would not allow absorption type sensor systems to be used since the mercury stream would not permit gamma radiation to pass through it.
In addition to the shortcomings of the prior art as discussed above, previously proposed systems including those described by Fearon and Lyman cause the sample to pass across a detector only once which imposes accuracy limitations.
In view of the foregoing, there is a need to provide a slurry ash monitoring system which enables rapid coal slurry evaluation and provides accurate evaluations of slurry composition. In accordance with the present invention, a closed slurry pumping and recirculating system is provided in which a batch of slurry is exposed to vacuum pressure conditions (i.e. pressure less than atmospheric). Exposure to low pressure causes air and other gas bubbles entrained within the slurry to collapse, thereby enabling radiation absorption type detectors to be used to provide accurate measurements without the above mentioned signal bias. Evacuating the gases enhances the accuracy of the measure of density of the sample and further removes dissolved gas which would otherwise absorb radiation and skew measurements when they are present in a dissolved state within the sample. Moreover, the physical configuration of the system according to this invention ensures that a homogeneous slurry mixture passes through the detector.
The system in accordance with this invention also causes the slurry sample batch to be passed through a detector multiple times which gives an average reading for the sample since each particle in the sample has several chances to attenuate the radiation signal. This approach provides enhanced accuracy in evaluating ash concentration. The present system enables slurry samples from multiple sources to be evaluated by a single instrument system.
Radiation absorption type ash analyzers according to the prior art evaluate the concentration of ash producing substances within the coal by evaluating the degree of absorption of a gamma radiation signal passing through the sample. This approach exploits the fact that the mass absorption coefficient for coal is significantly less than that for ash bearing minerals. In accordance with prior art approaches, measurements were taken for radiation of an energy level of 60 Kev or above. Unfortunately, at that energy level, the differences in mass absorption coefficient between coal and ash producing materials are not great, and therefore, these approaches must evaluate relatively small differences in absorption coefficient between these two types of material, giving rise to limitations in the accuracy of measurement. In accordance with this invention, the presence of ash producing materials is evaluated by examining the absorption of radiation at low energy levels, for example, in the 10-30 Kev range.
Irrespective of the energy level used to evaluate ash producing materials within a coal slurry, accuracy limitations are present since all ash producing materials do not behave alike in terms of radiation absorption. In particular, iron pyrite has a disproportionately high absorption characteristic. In accordance with this invention, emissions from the sample of X-ray energy at the 6.4 Kev level is measured and is a stimulated emission from iron pyrite. The measured concentration of pyrite is used to adjust the total ash measurements. Various approaches for measuring the radiation associated with pyrite are described herein. Measurements of iron concentrations in other materials such as iron ore could also be conducted in this manner.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.